This invention relates generally to oil and gas well logging tools. More particularly, this invention relates tools and methods for identifying the influx of formation fluids such as gas, oil, or water, into the borehole in real-time during drilling operations.
Exploration for hydrocarbons commonly includes using a bottomhole assembly including a drill-bit for drilling a borehole in an earth formation. Drilling fluid or “mud” used in the drilling may vary in density or “mud weight” for a number of reasons. Such variations can result from changes in the quantity and density of cuttings (particles of formation); changes in the “mud program” at the surface, changes in temperature, etc. Variations in mud density also occur when gas or liquid enter the borehole from the formation. Such influx of formation fluids may likely be the result of formation overpressures or abnormally high pressures.
Pressure detection concepts are especially important in drilling. Not only does the drilling rate decrease with a high overbalance of mud pressure versus formation pressure, but also lost circulation and differential pressure sticking of the drill pipe can readily occur. More importantly, an underbalance of mud pressure versus formation pressure can cause a pressure “kick.” A well may kick without forewarning. Balanced drilling techniques often require only a fine margin between effective pressure control and a threatened blowout. Additionally, there are situations where underbalance is maintained to avoid formation damage so that it is important to detect inflow of formation liquids into the borehole.
Some prior art techniques for detecting abnormal formation pressure are based on measurement of drilling parameters such as drilling rate, torque and drag; drilling mud parameters such as mud gas cuttings, flow line mud weight, pressure kicks, flow line temperature, pit level and pit volume, mud flow rate; shale cutting parameters such as bulk density, shale factor, volume and size of cuttings. All of these suffer from the drawback that the measurements are not available in real-time.
Other prior art methods for identifying possible kicks rely on density measurements of the borehole fluid. See, for example, U.S. Pat. No. 4,492,865 to Murphy et al, U.S. Pat. No. 4,412,130 to Winters, U.S. Pat. No. 6,648,083 to Evans et al., and U.S. Pat. No. 6,768,106 to Gzara et al. A drawback of methods that make density measurements is that gas must be present in sufficient quantities to affect the density of the mud, so that dissolved gas that may be a precursor to a gas kick would not register with density measuring devices. In addition, the density measurements made by the prior art devices are responsive to varying degrees to the density of the formation.
There is a need for a technique to measure the properties of the borehole fluid downhole with a single tool in order to detect kicks and inflow of formation liquids. The present invention satisfies this need.